Fixes#1696
Additional pin definitions for Pimoroni Tiny 2040, including using the Green LED element of the RGB LED as the default LED.
There are only 12 external header pins for GPIO, including 4 ADC pins, so there are less options for assigning pins than on a generic Pico RP2040. In particular, there can only be one SPI, and it can't have an SS pin but I've defined GPIO17, as on the pico, because the definition is used in generic / common.h.
BOOTSEL on the Pimoroni Tiny 2040 is connected to GPIO23. I don't know if that has any consequences for implementing reading from BOOTSEL for this board. That may need to be revisited, but it doesn't appear to involve any changes to pins_arduino.h.
